1. Field of the Invention
The present invention relates to a soft start control for an SCR and diode bridge and, more particularly, to a soft start circuit which provides phase control of the SCRs to limit in-rush current at start-up or during other periods.
2. Related Art
Conventional bridge rectifier circuits are not well equipped to handle high start-up currents and short circuits which stress circuit component such as fuses, bridge diodes and smoothing capacitors. Typically, for example, the occurrence of an output short circuit destroys a protection fuse between a power supply and the bridge rectifier, requiring replacement of the fuse. To limit the in-rush current at turn on, conventional circuits include a thermistor or a relay in series with the output of the rectifier bridge. Thermistor protection, although inexpensive, is suitable only for low power applications.
Relay protection, while more reliable and efficient than a thermistor, comes at a significant cost premium, typically 50% to more than 100% of the bridge rectifier cost, and occupies a significant amount of space, usually more than the bridge rectifier itself.
Therefore, a need exists for a self-contained bridge rectifier circuit which can be provided in a module or a package similar to that of a conventional bridge rectifier, but which also provides an in-rush current protection on start-up and short circuit protection during operation without a significant cost premium.
It is desirable to have a three-phase SCR rectifier bridge, with a Soft Start Integrated Circuit (IC) to provide phase-control of the SCRs to limit the charging current of a DC bus capacitor, regulate the DC output voltage in response to an input control voltage, and deliver status feedback signals.